Method and apparatus for calibrating cameras used in the alignment of motor vehicle wheels

ABSTRACT

An apparatus for determining the position and/or alignment of objects such as motor vehicle wheels, and including targets for attachment to the objects, a pair of optical sensing means such as television cameras for viewing the targets, an electronic processing means connected to the optical sensing means for processing data relating to images of the targets to determine position and/or alignment information, and a display means for displaying the position and/or alignment information. The optical sensing means view a target located on each object and form an image of each target. Electronic signals corresponding to each of the images are transferred to the electronic processing means which correlates the image signals of each of the targets with the true shape of each target. In so doing, the processing means relates the geometric characteristics and positional interrelationships of certain known elements of the target with the geometric characteristics and positional interrelationships of corresponding elements in the viewed images and calculates the position and/or alignment of the objects to which the targets are attached.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/544,378 filed Oct. 10, 1995, which is a continuation-in-partof U.S. patent application Ser. No. 08/122,550 filed Sep. 29, 1993. (nowU.S. Pat. No. 5,535,522), both of which are entitled "Method andApparatus for Determining the Alignment of Motor Vehicle Wheels" andassigned to the assignee of the present invention.

This application is also related to U.S. patent application Ser. No.07/940,935 filed Sep. 4, 1992 (abandoned), and International ApplicationNo. PCT/US93/08333 filed Sep. 3, 1993, both of which are entitled"Method and Apparatus for Determining the Alignment of Motor VehicleWheels" and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method and apparatus forcalibrating electronic cameras, and more particularly to a method andapparatus for determining the spatial position and relative orientationof a pair of electronic cameras used to optically determine thealignment of motor vehicle wheels.

2. Terms and Definitions

In the vehicle wheel alignment industry the following terms, withcorresponding definitions, are commonly used:

Camber is the angle representing the inward or outward tilt from truevertical of the wheel and is positive if the top of the wheel tiltsoutward.

Caster is the angle representing the forward or rearward tilt from truevertical of the steering axis. When a wheel is viewed from the side, theangle is positive when the upper ball joint (or top of king pin, orupper mount of a McPherson strut) is rearward of the lower ball joint(or bottom of the king pin, or lower mount of a McPherson strut).

Thrust Line (T/L) is a line that bisects the angle formed by the reartoe lines. Lines and angles measured clockwise from the 12:00 axis arepositive.

Geometric Center Line, is the line that runs from a point on the rearaxle midway between the rear wheels to a point on the front axle midwaybetween the front wheels.

Individual Toe is the angle formed by a front-to-back line through thewheel compared to the geometric center line. Angles pertaining to theleft side are positive when clockwise of the thrust line and anglespertaining to the right side are positive when counterclockwise of thethrust line.

Offset is the amount that a front wheel and its corresponding rear wheelare out of line with each other. If there is no offset, the rear wheelis directly behind the front wheel.

Setback is the amount that one wheel on one side of the vehicle isdisplaced back from its corresponding wheel on the other side of thevehicle.

Steering Axis is a line projected from the upper pivot point of theupper ball joint or top of kingpin, or McPherson strut, through thelower ball joint.

Steering Axis Inclination (SAI) is the angle between the steering axisand true vertical. If the steering axis appears to tilt inward at thebottom of the wheel (as viewed from the driver position), the SAI ispositive. SAI also is also known as kingpin inclination (KPI).

Thrust Angle (T/A) is the angle between the thrust line and thegeometric center line. Angles measured clockwise from the geometriccenter line are positive.

Total Toe is the sum of individual, side-by-side toe measurements. Iflines projected parallel to the primary planes of the wheels intersectat a point ahead of the side-by-side wheels, the angle is positive (toein). If the lines would intersect behind the side to side wheels, theangle is negative (toe out). If the projected lines are parallel, thetoe is zero.

Traditionally, the Camber and Toe measurements for each wheel of thevehicle are relative measurements i.e. relative to a vertical plane orto another wheel and these measurements are therefore made when thewheels are stationary. On the other hand, the calculation of Caster andSAI is a dynamic procedure and entails determining how the Camber of thefront wheels changes with respect to a change in steering angle. This isusually done by swinging the front wheels from left to right through anangle of between 10°0 and 30°, or vice versa, while determining theresultant changes in Camber of the wheel with steering angle changes.From these determinations the Caster and SAI are determined by methodswell known in the wheel alignment industry.

Similarly, once Camber, Toe, Caster and SAI have been measured, allother relevant wheel alignment parameters can be calculated by methodsand formulations well known in the industry.

3. Brief Description of the Prior Art

The wheels of a motor vehicle need to be periodically checked todetermine whether or not they are in alignment with each other because,if any of the wheels are out of alignment, this can result in excessiveor uneven wear of the tires of the vehicle and/or adversely affect thehandling and stability of the vehicle.

The typical steps of determining and correcting the alignment of avehicle's wheels are as follows:

1. The vehicle is driven onto a test bed or rack which has previouslybeen levelled to ensure a level base for the vehicle.

2. Some components of the alignment determination apparatus are mountedonto the wheels of the vehicle. These components are not necessarilyaccurately placed with respect to the wheel axis. The extent of theinaccuracy by which these components are mounted is called the "mountingerror".

3. A "runout" calculation is done by jacking the vehicle up and rotatingeach wheel and taking measurements of the orientation of that wheel atdifferent positions. These measurements are then used to calculate acorrection factor to compensate for the "mounting error" and actual rimrun-out.

4. A determination of the alignment of each of the wheels is done. Theresults of these determinations are compared to the specifications ofalignment parameters for the vehicle being tested.

5. The operator then adjusts the various linkages of each wheel tocorrect for the misalignment, if any, of the wheels.

6. Steps 4 and 5 are repeated until the alignment is up to standardand/or is within manufacturer's specifications.

A large variety of devices for measuring the alignment of a motorvehicle's wheels exist. Many of these use optical instrumentation and/orlight beams to determine the alignment of the wheels. Examples can befound in U.S. Pat. Nos. 3,951,551 (Macpherson); 4,150,897 (Roberts);4,154,531 (Roberts); 4,249,824 (Weiderrich); 4,302,104 (Hunter)4,311,386 (Coetsier); 4,338,027 (Eck); 4,349,965 (Alsina); 4,803,785(Reilly) and 5,048,954 (Madey).

All these devices operate with an apparatus which is mounted onto thewheel of a vehicle and which emits or reflects a light beam toilluminate an area on some form of reference such as a reference grid.As the position of the area illuminated by the beam on the reference isa function of the deflection of the beam, which in turn is a function ofthe orientation of the wheel, the alignment of the wheel can becalculated from the positioning of the illuminated area on thereference.

Other devices utilize a measuring head mounted onto each wheel of thevehicle. These heads typically include gravity gauges that are eitherconnected to adjacent heads by means of cords or wires under tension or,alternatively, configured with beams of light shining between adjacentheads. The measuring heads, which must be maintained level, are thenable to measure the relative angles between adjacent cords/beams oflight as well as the angles between each wheel and its adjacentcord/beam of light and, from these measurements, calculate the alignmentof the wheels.

Another type of alignment device is illustrated in U.S. Pat. Nos.4,899,218 (Waldecker) and 4,745,469 (Waldecker et al). This deviceoperates by projecting structured light onto a wheel of the motorvehicle so that at least two contour lines on the surface of the wheelare illuminated. These contour lines are then read by video cameraswhich are positioned offset from the optical plane of the structuredlight and which are connected to a processor which calculates thespatial position of the contour lines (and therefore that of the wheel)by means of triangulation.

Generally, the heads used in the above described wheel alignment devicesare delicate and expensive, complicated to use and must be carefully setup. Furthermore, certain of these devices rely on the accurate placingof optical or other measuring devices either on or in a set positionrelative to the wheels of the vehicle. This can be time consuming andcomplicated for the technicians operating the alignment determinationapparatus. Such equipment also has the disadvantage that componentswhich are carelessly left secured to the wheels when the vehicle ismoved from the test area can very easily be damaged. Such damage,particularly in the case of sophisticated equipment, can be costly.

German patent application DE 29 48 573 in the name of SiemensAktiengesellschaft discloses an apparatus which can be used to determineboth the orientation and the spatial position of the plane of the wheelof a motor vehicle as well as the three-dimensional position of thesteering axis of this wheel. The application discloses a method wherebya television camera takes an image of the rim on the wheel from twodifferent known height positions. These images are fed into a processorwhich relates them to the known coordinates and viewing angles of thecamera at its two height positions and determines the three-dimensionalposition of the rim.

In a similar way, a number of images of each wheel, in differentsteering positions, are taken to determine a three-dimensional solid ofrevolution for the wheel. From the axis of this solid of revolution thesteering axis of the wheel under investigation can be determined. As aresult, the three-dimensional position of both the steering axis and thecenter point of the plane defined by the rim of the wheel is determined.

In addition to the fact that little indication is given as to how theabove values are determined, the method and apparatus of the describedapplication has the disadvantage that, because a triangulation techniqueis used, at least two images (from different cameras or from a singlecamera viewing along different axes) of the wheel must be taken.Furthermore, both the coordinated three-dimensional position for eachpoint from where an image of the wheel is taken as well as theorientation of each of the view paths must be accurately known.

This is a major disadvantage of this invention because the accuratedetermination of the three dimensional positions and the orientation ofthe view paths, requires sophisticated equipment which can easily go outof calibration due to temperature changes, vibration, ground movement,etc.

A further disadvantage is that the method does not indicate how it makesallowances for the perspective distortion of the image of the rim of thewheel. This perspective distortion causes the image of the rim to be inthe form of a distorted ellipse with the edge of the ellipse closest tothe television camera appearing larger and the image of the edgefarthest from the camera appearing smaller. If allowance for thisdistortion is not made, inaccuracies can result.

The need therefore still exists for an optical wheel alignment apparatuswhich is simple and easy to use, which has its sophisticated alignmentdetection components remote from the wheels of the motor vehicle, andwhich can provide reliably accurate alignment measurements over a largerange of rim diameters, track widths and wheel bases.

A need also exists for a method and apparatus for facilitating thecalibration of the optical sensing devices used to determine wheelalignment and the like.

SUMMARY OF THE INVENTION

Objects of the Invention

It is therefore an object of this invention to provide a calibrationmethod and apparatus which is simple, easy and quick to use.

Another object of this invention is to provide a method and apparatusfor calibrating wheel alignment apparatus that uses an opto-electronicimage detection device to determine the alignment of vehicle wheels.

A further object of the present invention is to provide a rigid targetstructure that can be variously positioned in front of a dual cameraimage detection apparatus and used in association therewith to determinethe spatial position and relative orientations of the cameras.

Summary

Briefly a presently preferred embodiment of this invention includes arigid frame to which is mounted a pair of specially configured targets.The assembly is adapted for placement within the field(s) of view of atleast one of a pair of electronic cameras forming a part of anopto-electronic alignment system. The viewing camera(s) formsperspective images of each target, and electronic signals correspondingto each of the images are transferred to an electronic processing meanswhich correlates the perspective image of each of the targets with thetrue shape of each target. In so doing, the processor relates thedimensions of certain known geometric elements of the target with thedimensions of corresponding elements in the perspective image and byperforming certain trigonometric calculations (or by any other suitablemathematical or numerical methods), calculates the position of eachtarget relative to the viewing camera(s). By first positioning theassembly with both targets within the field of view of a single camera,and then positioning the assembly such that one of the targets is withinthe field of view of each camera, the processing means can determine theposition and orientation of each camera relative to the other. With suchinformation recorded, the system is "calibrated".

DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(c) are diagrams illustrating three different images of acircle resulting from various degrees of rotation about different axes;

FIG. 2 is a schematic representation illustrating a single camera systemused to inspect the alignment of motor vehicle wheels;

FIG. 2a is an illustration of a quasi three-dimensional representationof a type that may be generated on a system display screen to reportdetected alignment and to guide the technician in making appropriatevehicle adjustments;

FIG. 2b is a cross-section through a pan-and-tilt mirror used in oneillustrated target embodiment;

FIG. 3 is a representation of an exemplary target face design that canbe used with the apparatus in FIG. 2;

FIG. 4 is a schematic representation of an alternative embodiment of asingle camera apparatus;

FIG. 5 is a perspective view of an alternative target design mounted ona vehicle wheel;

FIG. 6 is a schematic representation of an image of the targetsillustrated in FIG. 5 formed using the optical system of FIG. 4;

FIG. 7 illustrates one method of how the apparatus calculates therun-out factor of the wheel;

FIGS. 8a-8c illustrate certain aspects of the mathematics performed inthe method and apparatus;

FIG. 9 is a diagram schematically illustrating a dual camera embodimentof an alignment system of the type described;

FIG. 10 illustrates details of the camera/light subsystem of FIG. 9;

FIG. 11 illustrates an alternative embodiment of a target array;

FIGS. 12a-12d are respectively side, top, front and back views of atarget assembly in accordance with the present invention; and

FIGS. 13 and 14 are perspective views showing the positioning of theassembly of FIGS. 12a-12d relative to dual cameras in implementing themethod of the present invention.

DETAILED DESCRIPTION

Basic Theory of Alignment System

This system is based on the fact that the image of a body variesaccording to the perspective from which such body is viewed and that thevariation in the image is directly related to and determinable from theperspective angle of the view path along which the body is viewed.

Furthermore it is known that it is possible to determine the perspectiveangles at which an object is viewed merely by relating the perspectiveimage of that object with a true non-perspective image thereof.Conversely put, it is possible to determine the angles at which anobject is orientated to a view path (or a plane perpendicular thereto)by comparing a perspective image of an object with a non-perspectiveimage thereof.

This is illustrated in FIGS. 1(a)-(c) with reference to a circle 10,shown as it would appear if viewed from three different perspectives. InFIG. 1(a) the circle 10 is illustrated as it would appear if it wereviewed along an axis perpendicular to its primary plane which, in thiscase, is in the plane of the paper. If this circle is rotated through anangle θ, being less than 90°, about the y-axis 12 and viewed along thesame view path, the image of the circle 10 will be that of an ellipse asshown in FIG. 1(b). Similarly, if the circle is rotated about both the xand the y-axes, 12 and 14 respectively, through angles θ and φrespectively, the image of the circle (the ellipse) will be as shown inFIG. 1(c), in which the major axis 16 of the ellipse is shown to beangled relative to both the x and y-axes.

It will, however, be realized that the ellipses here are idealized inthat they make no allowance for the distortion which results in an imagewhen it is viewed from a perspective angle. This distortion isillustrated by the broken lines 11 in FIGS. 1(b) and (c). As can be seenfrom theses Figures, the edge of the ellipse 11, which is closer to theviewer, appears larger while the edge 11, which is farther from theviewer, appears smaller. The resulting image 11 is thus a distortedellipse.

Returning to the idealized conditions shown in these figures, andassuming the angles θ and φ are not known, it is possible to determinethe orientation of the primary plane of the ellipse illustrated in FIG.1(c) by relating the image of the ellipse to the circle 10 in FIG. 1(a).This is usually done by relating the geometric characteristics (e.g.dimension) of at least one element of the ellipse (e.g. the major andminor axes 16, 18 thereof) to characteristics of corresponding elements(the diameters) of the circle in FIG. 1(a).

Under idealized conditions, these orientation calculations are done byapplying trigonometric functions or any other mathematical/numericalmethods to the ratios between the minor and/or major axis and thediameter. In addition, the angles of the minor and major axes to thehorizontal (x-) axis or vertical (y-) axis can be calculated. Once allthese angles have been determined, the orientation in space of theprimary plane of the ellipse will be determined.

Although not illustrated, it is also possible to determine the positionin space of the circle 10. This will, however, be demonstrated belowwith reference to FIG. 8.

The performance of the above illustrated calculations is complicated bythe real-life perspective distortion of the image, as illustrated by thebroken lines 11. How this foreshortening is allowed for will, onceagain, be discussed with reference to the mathematics illustrated inFIG. 8.

Brief Description of One Embodiment of the Alignment Apparatus

The apparatus with which this theory is applied is illustrated in theschematic representation in FIG. 2. In this figure a motor vehicle 20,on which a wheel alignment is to be performed, is represented by aschematic illustration of its chassis and is shown to include two frontwheels 22L and 22R and two rear wheels 24L and 24R. The vehicle 20 isshown positioned on a conventional wheel alignment test bed 26,indicated in dotted lines, which does not form part of this invention.

The alignment apparatus is shown to be constituted by a video camera 30which is in electrical communication with an electronic processing meanssuch as a computer 32 which, in operation, displays results andcalculations on a visual display unit 34. In addition, the apparatusincludes a keyboard 36 (or some other suitable means) for inputting dataand relevant information into the computer 32. It will, of course, beappreciated that display and keyboard entry could be provided by aremote unit which communicates with the computer through a cable,lightwave or radio link.

In accordance with an exemplary embodiment and as illustrated in FIG.2a, a computer-generated quasi three-dimensional representation of thewheels being aligned may be depicted on the display unit 34 along withsuitable indicia evidencing the detected alignment. In addition,alphanumeric and/or pictorial hints or suggestions may be depicted toguide the technician in adjusting the various vehicle parameters asrequired to bring the alignment into conformance with predeterminedspecifications.

The video camera 30 sights onto the wheels 22L, 22R, 24L and 24R along aview path 38 which passes through a lens 40 and onto a beam splitter 42.The beam splitter 42 splits the view path 38 into two components 38L and38R, respectively. As is apparent from this figure, the left handcomponent 38L of the view path 38 is reflected perpendicularly to theinitial view path by the beam splitter 42 while the right hand component38R is reflected perpendicularly to the initial view path by a mirror orprism 44 mounted adjacent to the beam splitter. The apparatus alsoincludes a housing 48 into which the beam splitter 42, mirror 44 and atleast two pan-and-tilt mirrors, 46L and 46R, are mounted. From thispoint onwards the respective components of the apparatus and the viewpath are identical for both the left and right side of the motor vehicleand therefore a description of only one side will suffice.

The left hand component of the view path 38L is reflected onto thewheels 22L and 24L by the left side pan-and-tilt mirror 46L which ismovable to allow the video camera 30 to consecutively view the frontwheel 22L and the rear wheel 24L of the vehicle 20. In some embodimentsof this invention the pan-and-tilt mirror 46L can be configured so thatboth the front and rear wheels of the motor vehicle can be viewedsimultaneously.

In this embodiment, the view path 38L passes from the pan-and-tiltmirror 46L through an aperture 50L in the wall of the housing 48 andonto the respective wheels 22L and 24L. A shutter 52L is positioned sothat it can be operated to close the aperture 50L thereby effectivelyblocking the view path 38L and allowing the video camera 30 to sightonto the right hand side of the vehicle 20 only. Alternatively, shutterscould be placed at the locations 53L and 53R and/or an electronicshutter within the camera 30 could be synchronized with one or morestrobed light sources to permit capture of an image only when aparticular target or targets are illuminated.

Operation of the Alignment Apparatus

In a typical operation, the apparatus of this embodiment of theinvention works as follows: The vehicle 20 is driven onto the test bed26 which basically consists of two parallel metal strips on which thewheels of the vehicle rest. Under the test bed, a lift mechanism islocated (but not shown) which acts to lift the metal strips and thevehicle to allow the wheel alignment technician to access the wheelmountings to correct misalignment of the wheels. In addition, arotationally mounted circular plate commonly called a turnplate (notshown), is located under each front wheel of the vehicle. The turnplatesallow the front wheels to be pivoted about their steering axesrelatively easily. This facilitates the procedure involved during thecalculation of caster and other angles determined dynamically. The rearwheels are positioned on elongate, rectangular, smooth metal platesmounted on the metal strips. These plates are usually termed skid platesand allow the rear wheels to be adjusted by a technician once the rearwheel mountings have been loosened. Such plates also prevent preload towheels tending to affect their angular position.

In addition, as in some sophisticated alignment machines, the vehiclemake and model year can be entered into the apparatus at some time earlyon in the procedure, and this information is used by the apparatus todetermine the alignment parameters, for the vehicle concerned, frompreviously programmed lookup tables within the computer 32. Furthermore,from the vehicle's make and model year, the track width and wheelbasedimensions can be determined by retrieving the data from memory. Thesecan be used to drive the mirrors of the alignment apparatus to "home" inon the wheels of the vehicle more accurately. Alternatively, previousoperational history information can be used to select likely wheellocation. Still another possibility is to cause the mirrors to sweep aparticular pattern.

Once the vehicle 20 has been driven onto the test bed 26, a target 54 ismounted onto each wheel. The shape and configuration of the target willbe described later with reference to FIG. 3. The apparatus first makes a"run-out" factor calculation according to the method that will morefully be described with reference to FIG. 7.

Once the "run-out" factor has been calculated, the alignment apparatusforms an image (a detected image) of each of the targets 54 on thewheels of the motor vehicle 20. These detected images are processed inthe electronic processing means/computer 32 which calculates, using themethod of the invention as will be more fully described, the orientationof each of the targets to the respective view paths 38L, 38R. Thecomputer 32 then takes into account the "run-out" factors mentionedabove to calculate the true orientation of the wheels relative to therespective view paths. Thereafter the apparatus makes allowance for theorientation of the pan-and-tilt mirrors 46L, 46R to calculate the actualorientation of the primary planes of each of the wheels. Upon this beingdone, the results of the computation are displayed on the display 34which gives the operator the required instructions as to whichcorrections need to be made to, for example, adjustments to the steeragelinkages 60 of the front wheels 22L and 22R to correct the detectedmisalignment of the wheels of the vehicle.

Orientation Calculations

The computer 32 does all the required calculations using a computerprogram such as IMAGE ANALYST, which is capable of analyzing images andvalues associated therewith. Typically, IMAGE ANALYST produces valuesfor the center points of these images in coordinates relating to thepixels on the screen of the video camera. These values are thenprocessed by software which incorporates the later-to-be-describedmathematics illustrated with respect to FIG. 8. Although software suchas IMAGE ANALYST may have many features, in this application it isapparent that the main features utilized in this application is that ofbeing able to provide screen coordinates for the images detected by thevideo camera. It is, therefore, possible for software other than IMAGEANALYST to be used with this method and apparatus. IMAGE ANALYST issupplied by AUTOMATIX, INC. of 755 Middlesex Turnpike, Billerca, Mass.01821.

Orientation of the Pan-and-Tilt Mirrors

In the above-described method it is evident that knowledge of theorientation of the pan-and-tilt mirrors 46L, 46R is required for theeffective calculation of the relative alignment of the wheels of thevehicle 20 to each other. The orientation of these mirrors 46L, 46R canbe determined in one of two ways. One way of determining the orientationis by linking the mirrors 46L, 46R to a sensitive tracking andorientation determination device which outputs data to the computer 32which, in turn, calculates the orientation of the mirrors inthree-dimensional space. Alternatively, and preferably, the face of eachmirror includes a clearly defined pattern, usually in the form of anumber of small, spaced-apart dots, which define an identifiable patternthat can be detected by the video camera 30 as it sights onto the wheelsof the motor vehicle 20. Once the video camera 30 has detected thepattern on the mirrors 46L, 46R it can form an image thereof; an imagewhich, because of the orientation of the mirrors, will be a perspectiveimage, and which can then be electronically fed into the computer which,in turn, can calculate the minor orientation in three-dimensional spacealong the same lines as the orientation of the wheels of the vehicle 20are calculated. This second alternative is preferable because it doesnot require sophisticated and expensive electronic tracking andorientation determination equipment.

One way of implementing this second, preferable alternative, is toincorporate a lens 40 into the apparatus. The lens has a focal lengthsuch that it projects an adequately clear image of both the targets andthe mirrors onto the camera 30.

In FIG. 2b, one way of enhancing the images of the dots on thepan-and-tilt mirrors is illustrated. This figure illustrates across-section through a pan-and-tilt mirror 46L with two dots 41 shownformed on its upper surface. A plano-convex lens 43 is located on top ofeach dot. The focal length of each of these lenses is such that,together with the lens 40, they form a clear image of the dots in thevideo camera 30. Although this figure illustrates two individualplano-convex lenses 43, it will be evident that a single lens spanningtwo or more dots could be used. Similarly, other optical methods can beused to accomplish this.

Orientation of the Targets

An example of a typical target 54 that can be used on the wheels of thevehicle 20 is illustrated in FIG. 3. As can be seen from this figure,the target consists of a flat plate with a pattern of two differentlysized circles 62, 63 marked in a pre-determined format thereon. Althougha specific pattern is shown in this figure, it will be evident that alarge number of different patterns can be used on the target 54. Forexample, the target need not be circular, a larger or smaller number ofdots may be included. Moreover, other sizes and shapes can be used forthe dots. In addition, multifaceted plates or objects can also be usedfor the targets.

In practice, a mathematical representation, or data corresponding to atrue image (i.e. an image taken by viewing the target perpendicularly toits primary plane) and the dimensions of the target are preprogrammedinto the memory of the computer 32 so that, during the alignmentprocess, the computer has a reference image to which the viewedperspective images of the targets can be compared.

The way that the computer calculates the orientation of the target 54 isto identify certain geometric characteristics on the target 54, takeperspective measurements of these and compare these measurements withthe true image previously pre-programmed into the memory of thecomputer.

The apparatus could, for example, calculate the center of each of thecircles 62a, 62b by means of, say, a method called centroiding. This isa method commonly used by image analysis computers to determine thepositioning of the center point or center line of an object. Once thecenter points of the two circles 62a, 62b have been determined, thedistance between the two can be measured. This process is then repeatedfor other circles in the pattern on the target 54. These distances canthen be compared to the true distances (i.e. non-perspective distances)between the respective centers. Similarly, the angle to the horizontal(or vertical) of the line joining the two centers can be determined.Once allowance has been made for the effect of the focal length of thelens 40 and other optical characteristics of the components, such asbeam splitter 42, mirror 44 and mirrors 46L, 46R, are considered, acalculation can be made as to what the orientation of the target 54 is.This calculation can be done by using trigonometric functions or othersuitable mathematical or numerical methods. As explained above, thiswill also yield the orientation of the primary plane of the wheel of thevehicle.

Although the above describes one method of calculating the orientationof the target 54, it will be evident that other methods are alsoavailable. For example, the apparatus could sight onto only one of thecircles, say the circle 63, and by using the perspective image thereof(the distorted ellipse) calculate, in very much the same way asdescribed with reference to FIG. 1, the orientation of that circle and,therefore, the orientation of the target 54. Another example is to taketwo images rotated about 60° relative to each other and use suchinformation to calculate the orientation of the target with respect toits axis of rotation. Note that only two images are required so long asthe wheel axle does not change its axial orientation. In addition, it isenvisaged that in sophisticated alignment systems more than onecalculation will be completed for each target and that the differentresults of these calculations will be compared to each other to ensurethe required accuracy.

Furthermore, as the true dimensions of the target are preprogrammed intothe memory of the computer 32, the method and apparatus of thisinvention can be used to determine the exact position of the wheels inthree-dimensional space. This can be done by firstly determining theperspective image of certain of the elements of the pattern on thetarget (for example, the distances between circles) and comparing thedimensions of this image to the true dimensions of those elements. Thiswill yield the distance that the element and, accordingly, the target 54is from the video camera.

As the processes described above have already yielded the orientation oftarget 54 with respect to the view path and/or some other referenceplane, this result can be combined with the calculated distance and thegeometric coordinates of the alignment apparatus to yield a position ofthe target 54 relative to the alignment apparatus. During thiscomparison process, the effect of the focal length of the lens 40, aswell as the optical characteristics of the beam splitter 42, mirror 44and the pan-and-tilt mirrors 46L and 46R must also be taken intoconsideration. Typically, these characteristics would be input into thecomputer by direct entry or, preferably, by calibration techniques. Inthis way the exact positioning of each of the wheels of the vehicle 20can be calculated.

Brief Description of an Alternative Embodiment of the AlignmentApparatus

It will be evident to one skilled in the art that a number of differentconfigurations of lens, beam splitter and mirrors (i.e. the opticalsystem) are possible to achieve the desired result with the illustratedmethod and apparatus. One such configuration is illustrated in FIG. 4 ofthe accompanying drawings.

In this figure the equipment is shown to be suspended over the motorvehicle 20 and includes a video camera 30, computer 32 with associateddisplay 34 and data entry keyboard 36 as well as lens 40 similar tothose illustrated in FIG. 2. As with the configuration in FIG. 2, theview path or optical center line of the video camera 30 is deflectedinto two directions 38L and 38R by a combination of beam splitter 42 andplane mirror 44.

This configuration also includes two pan-and-tilt mirrors 70L, 72L,located on the left side of the apparatus and two pan-and-tilt mirrors70R and 72R located on the right side of the apparatus. The mirrors 70L,72L are arranged to view the left front and left rear wheels 22L, 24L,respectively and the mirrors 70R, 72R are arranged to view the rightwheels 22R, 24R respectively. As the mirrors 70L, 72L, 70R, 72R arepan-and-tilt mirrors, they can be moved to view the wheels on thevehicle 20 even though the vehicle is not accurately centered below theapparatus. These mirrors are also useful in making allowance forvehicles of different lengths of wheelbase and track width.

A further modification of this apparatus would include the replacementof the beam splitter 42 and the plane mirror 44 with a single reflectingprism. The prism has the advantage over the beam splitter combination inthat more light is reflected from the prism into the camera 30. Thisresults in a brighter image of the target 54 being formed by the camera30.

Target and Target Image Details

With the apparatus as illustrated in this figure, as with the otherillustrated apparatus, a modification of the target, as indicated inFIG. 5, can be used. In this figure the target, generally indicated as80, is shown to include a flat, rectangular plate 82 which is clamped tothe rim 84 of a wheel 86 by means of a clamping mechanism 88. It will beevident from FIG. 5 that the plate 82 is angled relative to the primaryplane of the wheel 86 as well as to its axis of rotation 89.

The precise orientation of this plate 82 relative to the wheel axis is,however, not known and will, as is described later, be computed withrespect to the wheel axis by the determination of a run-out factor forthis wheel. The general orientation of the plate 82 is, however, chosenso that it can be adequately viewed by the video camera 30 as it sightsonto it.

Finally, plate 82 includes a plurality of dots 90 which, as shown,constitute a pattern not unlike that on the target illustrated in FIG.3.

With targets of this nature, the images formed by the video cameras 30,when used together with the apparatus illustrated in FIG. 4, will besomething like that illustrated in FIG. 6. In this figure, it isapparent that four discrete images 92, 94, 96, 98 are formed to make upthe complete image, generally indicated as 99, formed by the videocamera 30. Each of the four images that make up the complete image 99 isan image of one of the rectangular plates 82, respectively disposed onthe four wheels of the motor vehicle. For example, the image 92 at thetop of the picture 100 could correspond to the plate 82 on the rightrear wheel 24R of the vehicle 20. Similarly, image 94 could correspondto the right front wheel 22R, image 96 to the left front wheel 22L andimage 98 to the left rear wheel 24L.

The advantage of the target 80 when used with the apparatus illustratedin FIG. 4 is that a single image can be taken simultaneously of all fourwheels. This single image can then be processed, in very much the sameway as described above, to yield the orientation and location of all thewheels to each other. More particularly, the relative orientation of theright front wheel to the left front wheel and the right rear wheel tothe left rear wheel can be calculated.

On either end of the images 92, 94, 96, 98 a pair of dots 100 can beseen. These dots 100 are in fact images of the dots on the respectivepan-and-tilt mirrors referred to in the discussion of FIG. 2. As waspointed out in that discussion, these dots are used to calculate theorientation of the pan-and-tilt mirrors to the view path of the camera;a calculation which is essential to determine both the orientation andthe location of the primary plane of each of the wheels of the vehicle.

In addition, this figure illustrates that the images of the marks 100can be separated from the images of the patterns on the plate by meansof a vertical line 101. This line 101 serves as a demarkation linebetween the pattern (from which the orientation of the target iscalculated) and the image of the dots 100 (from which the orientation ofthe pan-and-tilt mirrors is calculated).

Runout Factor Calculations

In FIG. 7 of the drawings, a method of calculating the run-out factorfor a target 104 mounted in a slightly different way on a wheel 103 isillustrated. In this method, the wheel 103 is slowly rotated while anumber of different images of the target 104 are taken. This target is,for the sake of clarity, off-set fairly substantially from the center ofthe wheel. In practice, however, the target may be mounted closer to thecenter, much like the target illustrated in FIG. 5. For each image, theinclination of the plane of the target, as well as its location in spaceis calculated. Once these have been determined for each image, they areintegrated to define a surface of revolution 106. This surface ofrevolution 106 will represent the path which the target 104 tracks asthe wheel is rotated about its axis, and the axis of rotation 108thereof is the same as the axis of rotation of the wheel. This meansthat a plane perpendicular to the axis of rotation 108 of the surface ofrevolution 106 will be parallel to the primary plane of the wheel 106.As the surface of revolution 106 is determined, its axis of rotation 108is determined and, therefore, the orientation and position in space ofthe primary plane of the wheel of the vehicle can be determined.

From these results, the run-out factor can be determined by calculatingthe angle between the plane of the target and the primary plane of thewheel. This run-out factor is then stored in the computer 32 and usedwhen the alignment of the wheel is calculated from a single image of thetarget.

The calculation of the run-out factor can also be used to determinewhether or not the suspension of the vehicle is badly worn. Using themethod of the invention an apparent run-out factor (i.e., theorientation of the target with respect to the wheel) can be determinedfor each image which is taken of the target. From this group ofindividual run-out factors a mean value can be calculated (which willrepresent the true "run-out" factor) as well as the extent of thedeviation from the mean of the individual factors. If this deviation isabove a certain tolerance, this indicates that the suspension of themotor vehicle is worn so badly that it needs to be attended to.

Accuracy Determination

Turning once again to the targets, it should be realized that animportant feature of the target illustrated either in FIG. 3 or 5 (orany other target for that matter) is that it should have sufficient datapoints to allow redundant calculations to be made using different setsof data points. This will yield multiple wheel alignment angles whichcan be averaged out to improve the accuracy of the final measurement. Inaddition, a statistical distribution of the different alignment anglescalculated for each wheel can be used as a measurement of accuracy ofthe operation of the apparatus. If a suitable check is built into thecomputer 32, a statistical distribution such as this can enable thecomputer 32 to determine whether or not sufficient accuracies exist and,if not, to produce a signal which can alert an operator to this fact.

Similarly, if the above checking indicates that one or more of thetargets used yield(s) unacceptably poor results while the remainingtarget(s) yield acceptable results, it can be assumed that some of thetargets being used are unacceptable. The computer can give an indicationto this effect and the operator can, for example, be instructed toremove, clean or repair the offending target(s).

A further benefit derived from forming suitable multiple images andcomputing a statistical analysis, is that the computer 32 can determinewhether or not enough images have been taken to suitably ensure therequired accuracy of the alignment measuring process. If insufficientreadings exist, the computer can direct the apparatus to take furtherreadings which, although sacrificing speed, would result in improvedaccuracy of the measurement.

Furthermore, the target could include a machine-readable, e.g. a barcode or the like, which can be used for identification, target tracking,intensity threshold measurement, evaluation of illumination quality, andencoding of defects to allow the use of cheap targets. For example, ifthe target was twisted and the amount of twist was encoded in the barcode, then the computer could compensate for the twist.

Another important feature of the target is that the pattern thereonshould allow very quick and accurate location of the pattern to anaccuracy approaching substantially less than a camera pixel. To achievethis the pattern should exhibit a high contrast and be of aconfiguration which allows the specific apparatus used to achieve therequired speed and accuracy. In one embodiment, retro-reflectivematerials are used for the dots, and a color that is absorptive of theparticular light used is chosen for the background.

This apparatus also allows for calibration, which is important as alloptical systems have some geometric distortion. The total image area ofthe apparatus could, for example, be calculated using a perfect targetand the result used to determine correction values that can be storedfor use when operating the system in alignment procedures.

The absolute accuracy of the apparatus can be checked or calibrated byusing a simple 2-sided flat plate target which is placed so that theapparatus views both sides simultaneously. As the plate is flat, the netangle (relative alignment) between the two planes of the target shouldbe zero. If not, a suitable correction factor can be stored in thecomputer. Alternatively, two views of the same side of the target takenfrom different angles could be used for this purpose.

Mathematical Algorithms Used

This section provides the mathematics necessary to reduce measurementsmade by the video camera to wheel positions in space using instantaneousmeasurement.

Assumptions

The camera system can be defined to include two planes positionedarbitrarily (within reasonable constraints of visibility) with respectto one another. One is the image plane, which maps what is "seen" by thecamera and the other is the object plane, which containsthree-dimensional, essentially point targets.

Based on this, the assumptions made are:

(i) the camera principal axis is normal to the image plane (most camerasare built this way);

(ii) there exists, at a known distance of f (i.e. the imaging system'sfocal length when set at infinity) from the image plane, along thecamera principal axis, a point called the center of perspectivity (CP)such that the behavior of the camera is that the image of a viewed pointanywhere in the camera's field of view is to project it onto the imageplane by moving it along a line passing both through the viewed point inspace and the CP;

(iii) the origin of the coordinate system fixed in the image plane islocated at the center of perspectivity, with z unit vector directedtoward camera along its principal axis; and

(iv) the units of the image plane measurements are the same as those ofthe object plane measurements.

These assumptions are commonplace in the visual sciences.

Overview

For this configuration, mathematics can be provided to determine therelative orientations and positions of the object and image planes.

This mathematics can be used in 2 ways:

(i) during calibration, to find the position of the image plane withrespect to the location of an object plane of known position of acalibration target; and

(ii) during the alignment process, to find the position and orientationof the primary plane of the target mounted on the wheels of the vehicle.It is essential in this step that the known coordinate system is fixedin space, and that it remains the same for all four wheels of the car.

As has been described above, once the location of the target planes onthe wheels is known, by rotating the wheels, the axis of rotation of thewheels can be determined, and from there, the alignment of the wheels.

Main Algorithm

It should be noted that this main algorithm presents no treatment of thevarious pan-and-tilt mirrors; this is done later.

The main algorithm requires the following inputs:

(i) A list of points expressed in object plane coordinates.

    .sup.o q.sub.j= (x.sub.j, y.sub.j), j=1,n/n≧4

These are actually three-dimensional points, but the object planecoordinate system can always be chosen so that the third coordinatez_(i) =0.

(ii) A corresponding list of image plane point coordinates ^(i) q_(j)=(u_(j), v_(j)), j=1, n.

For these inputs, the algorithm produces an output which is ahomogeneous coordinate transform matrix expressing the center ofperspectivity and unit vectors fixed with respect to the principal axesof the image plane. This matrix will normally be inverted, and thenapplied to transform the viewed points into image system coordinates.

Step 1: Determine a Collineation

Convert all the two-dimensional input coordinates to affine form andfind a 3×3 transformation matrix T such that: ##EQU1## for i=1, n andwhere the k_(i) are arbitrary scalar constants.

One way in which the transformation matrix T can be determined is givenbelow.

Step 2: Determine transforms of key points and invariants.

The transform matrix T will transform points in the object plane topoints in the image plane under the projectivity whose center is thecenter of perspectivity (CP). When inverted, it will also perform thereverse transformation, viz: ##EQU2##

It will be noted that the whole equation may be multiplied by anarbitrary scalar and still remain true. The value m_(i) is such ascalar, and is required to permit normalization of (u_(i) v_(i) 1)^(T)So that its third coordinate is a unit. The matrix T is also useful fortransforming lines, which are dual to points on the projective plane.The equation of a line in the projective plane is: ##EQU3##

Where c is the coordinate vector of the line and X is the specimenvector. Any homogeneous representation of a point which satisfiesequation 3 lies on the line. Suppose that an object co-ordinate ^(o) clies on a line, then: ##EQU4## is the equation of a line in the objectplane, expressed in object plane coordinates. Using equation 2 we cantransform to image plane co-ordinates: ##EQU5##

Therefore ##EQU6## is the way to transform line coordinates from theobject plane to the image plane and ##EQU7## is the way to perform theinverse transformation.

Note that the projective plane differs from the non-projective plane inthat it includes points at infinity whose projective coordinate is 0.These points together constitute a line at infinity, whose coordinatesare 0,0,1! viz. ##EQU8## This is illustrated in FIG. 8a which representsa side view of an object plane OP and image plane IP positionednon-parallel to each other at some angle θ.

The object plane OP intersects a plane parallel to the image plane IPbut passing through the center of perspectivity CP. This plane is calledthe view image plane VIP and intersects the object plane OP at the"vanishing line" mapped to the object plane, shown as point VLO.Similarly, the figure shows a plane parallel to the object plane calledthe viewed object plane VOP which intersects the image plane IP at a"vanishing line" mapped to the image plane, shown as point VLI.

As VIP is parallel to IP they intersect at infinity. The collineationmatrix T can therefore be used to map the line at infinity of the imageto its transformed position in the object plane as follows: ##EQU9## andlikewise: ##EQU10##

By the assumptions stated above with respect to the camera system, thecoordinates of the principal point of the image PPI are: ##EQU11##

The coordinates of the principal point of the object PPO are: ##EQU12##Step 3: Complete Remaining Inclination Values

The minimum distance between a line in a projective plane with linecoordinates z₁ z₂ z₃ !^(T) and a point with coordinates P₁ P₂ P₃ !^(T)is given by ##EQU13## This makes it possible to solve for DI, θ and DO:##EQU14## Step 4: Compute Pan Values

FIG. 8b illustrates a plan view of the object plane, looking down fromthe center of perspectivity.

We have ##EQU15## Step 5: Solve for Remaining Unknowns

Referring to FIGS. 8a and 8b together: ##EQU16## This is the origin ofthe image plane coordinate system expressed in object plane coordinates.It is located at CP.

^(o) _(x).sbsb.i and ^(o) _(y).sbsb.1 the remaining unit vectors can becomputed by transformation of the corresponding unit vectors in theimage plane, and subsequent orthogonalization with respect to z₁.##EQU17## can be orthogonalized with respect to ^(o) _(z).sbsb.i

    .sup.o.sub.x.sbsb.i =.sup.o.sub.x.sbsb.i -(.sup.o.sub.x.sbsb.i ·.sup.o.sub.z.sbsb.1) .sup.o.sub.z.sbsb.f        (25)

and renormalized ##EQU18## Similarly, let ##EQU19## Finally ##STR1##

Frames transform from image space to object space is the frame toreturn, and to express points given in the object plane coordinates interms of the coordinate system fixed with respect to the image plane, wenote ##EQU20##

That is the general case, but there is also the special case when theobject and image planes are parallel. This is detectable when VLO or VLI(equations 9 or 10) turn out to lie at infinity themselves (meaningtheir first two coordinates lie sufficiently close to 0).

In this case, ##EQU21## and the distance DCP can be determined by takingany point in the object plane (X_(k), Y_(k)) whose corresponding (u_(k),v_(k)) is not zero and calculating according to the diagram in FIG.8(c): ##EQU22## and then proceeding as from equation (22).

This concludes the description of the main algorithm to determine planedisplacements.

9.4 Determination of Transform Matrix

This section illustrates how to calculate the transform matrix T used inequation (1).

The method presented here is an analytic method which maps between only4 coplanar points and is based on the fundamental theorem of projectivegeometry which tells us that given four points in the projective plane:

    p.sub.1 =(x.sub.1 y.sub.1 w.sub.1)                         (38)

    p.sub.2 =(x.sub.2 y.sub.2 w.sub.2)

    p.sub.3 =(x.sub.3 y.sub.3 w.sub.3)

    p.sub.4 =(x.sub.4 y.sub.4 w.sub.4)

constants c₁, c₂ and c₃ can be found such that

    p.sub.4 =c.sub.1 p.sub.1 +c.sub.2 p.sub.2 +c.sub.3 p.sub.3 (39)

When this is represented in matrix form: ##EQU23## then the matrix Mconsisting of ##EQU24## will transform the ideal points origin and unitpoints as

follows: ##EQU25## Therefore, to construct a transform which maps fourarbitrary points p₁, p₂, p₃, ₄ to four arbitrary other points q₁, q₂,q₃, q₄, two transforms must be constructed: ##EQU26## and then M, suchthat

    q.sub.i =p.sub.i M                                         (44)

is given by

    M=M.sub.i .sup.-1 M.sub.2 (45)

Note that in this section, the p's and q's are now vectors. In the mainsection, column vectors are used, so

    T=M.sup.T                                                  (46)

Finally, another method (not illustrated here) accepts more than fourpoints and does a least-squares approximation using pseudo-inverses.This second method can be used in the case where the number of pointsmeasured has been increased to compensate for expected errors.

Allowance for Pan-and-Tilt Mirrors

After the imaged data points have been converted back tothree-dimensional points given in image plane coordinates, it remains tomake allowance for reflections by the beam splitter assembly and thepan-and-tilt mirrors.

If ^(i) x is a point to be reflected, and ^(i) n is a unit-length normalto the plane of reflection, ^(i) x_(o) is a point in the plane ofreflection (all expressed in image-plane coordinates) then ^(i) x_(r),its reflection is given by ##EQU27## The matrix above is a standarddisplacement style transform which may be inverted using standardmethods though there is no need to do so in the present application.These matrices may also be cascaded as usual from right to left, to dealfirst with the beam-splitter and then with the pan-and-tilt mirror, butthe reflection plane point ^(i) x_(o) and normal ^(i) n for thepan-and-tilt mirror must first be transformed by the beam-splitterreflection matrix before the pan-and-tilt mirror reflection matrix isformed from them.

Finally, it should be noted that when the main algorithm is used to findthe position of the pan-and-tilt mirror, once these have been reflectedthrough the beam splitter. ^(i) _(z).sbsb.0 and ^(i) 0₀ are directlyusable as normal and point in the reflection plane directly.

A subsequent use of an iterative fitting procedure may result inimproved accuracies.

Other mathematical processes can also be used to process the imagesdetected using the apparatus.

Alternative Two-Camera Embodiment

In FIG. 9 of the drawing, an alternative embodiment of the alignmentapparatus utilizing a pair of fixed, spaced-apart cameras is depicted at110. A four-wheeled vehicle positioned on a lift ramp 111 for wheelalignment is suggested by the four wheels 112, 113, 114, and 115. In theusual case, the rack 111 will include pivot plates (not shown) tofacilitate direction change of at least the front wheels. In thisembodiment a camera supporting suprastructure includes a horizontallyextending beam 116 affixed to a cabinet 117. The cabinet 117 may includea plurality of drawers 118 for containing tools, manuals, parts, etc.,and may also form a support for a video monitor 119 and input keyboard120.

Mounted at each end of the beam 116 is a camera and light sourcesubsystem respectively designated 122 and 124. The length of beam 116 ischosen so as to be long enough to position the camera/light subsystemsoutboard of the sides of any vehicle to be aligned by the system. Thebeam and camera/light subsystems 122, 124 are positioned high enoughabove the shop floor 125 to ensure that the two targets on the left sideof the vehicle are both within the field of view of camera assembly 122,and the two targets on the right side of the vehicle are both within thefield of view of camera assembly 124. In other words, the cameras arepositioned high enough that their line of view of a rear target is overthe top of a front target. This can, of course, also be accomplished bychoosing the length of beam 116 such that the cameras are outside of thefront targets and have a clear view of the rear targets. Details of thecamera/light subsystems 122, 124 are discussed below with respect toFIG. 10.

In accordance with this embodiment, a target device 126, including arim-clamp apparatus 128 and a target object 130, is attached to eachwheel. A suitable rim-clamp mechanism is discussed in U.S. Pat. No.5,024,001 entitled "Wheel Alignment Rim Clamp Claw". As will bedescribed in more detail below, the preferred target object has at leastone planar, light-reflective surface with a plurality of visuallyperceptible, geometrically configured, retro-reflective target elements132 formed thereon. Such target surfaces may be formed on one or moresides of the target object. In use, each target must be positioned on avehicle wheel with an orientation such that the target elements arewithin the field of view of at least one of the camera/light subsystems.

In FIG. 10 of the drawing, further detail of the camera and lightingcomponents is illustrated. Mounted within the partially broken-away endof beam 120, the subsystem 122 is shown to include a lighting unit 140,comprised of a plurality of light emitting diode (LED) light sources 142arrayed about an aperture 144 through which the input optics 146 of asuitable video camera 148 is projected. The light array in the preferredembodiment includes 64 LEDs (a lesser number being shown for simplicityof illustration) which provide a high-intensity source of on-axisillumination surrounding the camera lens, to ensure that maximum lightis retro-reflected from the targets. In order to discriminate againstother possible sources of light input to the camera 148, a narrow bandfilter matched to the light spectrum of the LEDs may be positioned infront of the lens 146.

Although any suitable type of video camera can be utilized, inaccordance with the preferred embodiment a CCD device is utilized. Thiscamera has a resolving power suitable for the present application.

In FIG. 11, an example of a target in accordance with a preferredembodiment is depicted and includes a plurality of light-reflective,circular target elements or dots of light-colored or whiteretro-reflective material disposed in an array over a less reflective ordark-colored surface of a rigid substrate. Suitable retro-reflectivematerials include Nikkalite™ 1053 sold by Nippon Carbide Industries USA,Scotchlite™ 7610 sold by 3M Company, and D66-15xx™ sold by Reflexite,Inc.

The target includes multiple circular dots so as to ensure thatsufficient data input may be grabbed by the camera even in the case thatseveral of the target elements have been smudged by handling or areotherwise not fully detectable. In accordance with the preferredembodiment a well defined target includes approximately 30 circular dotsvery accurately positioned (within 0.0002") with respect to each other.By way of specific example, the target illustrated in FIG. 11 mightinclude 28 circular dots of 1" diameter very accurately positioned on a2"×2"grid, with four 11/4" dots and a single 11/2" diameter dotstrategically positioned within the array. By mathematically moving themathematical image of a target until the mathematical position andorientation of the dots line up with the dots of the real target in thereal image, position and orientation information can be obtained. Thismathematical manipulation of a well defined target until it is orientedthe same way as the image is called "fitting the target." Once thefitting is accomplished, the position and orientation of the target isvery accurately known (to within 0.05" and 0.005°). Such accuracy isobtainable because the target is made to very strict tolerances andbecause the design enables measurement of many points (1,500 measuredpoints, i.e. 30 or so fiducials (dots) each with detected 50 edgepoints). Furthermore, the use of subpixel interpolation enhances theaccuracy of measurement to beyond the pixel resolution of the CCDcameras.

The target is typically manufactured using a photolithographic processto define the dot boundaries and ensure sharp-edge transition betweenlight and dark areas, as well as accurate and repeatable positioning ofthe several target elements on the target face. The target face may alsobe covered with a glass or other protective layer. Note that since allinformation obtained from a particular target is unique to that target,the several targets used to align a vehicle need not be identical andcan in fact be of different makeup and size. For example, it isconvenient to use larger rear targets to compensate for the differencein distance to the camera.

In order to accurately determine the position between the wheels on oneside of the vehicle and the wheels on the other side of the vehicle, thesystem must know where one camera is positioned with respect to theother camera. This is accomplished during a calibration and set upoperation wherein, as depicted in FIG. 9, a larger target 150 (presently3'×3') is positioned in the field of view of both cameras, typicallyalong the centerline of the rack 111, and the approximately 30 feet awayfrom the cameras. Information obtained from each camera is then used todetermine the relative positions and orientations of the cameras. Morespecifically, since each camera will indicate where the target is withrespect to itself, and since each is viewing the same target, the systemcan calculate where each camera is located and oriented with respect tothe other. This is called a relative camera position (RCP) calibration.Such calibration allows the results obtained from one side of thevehicle to be compared to the other. Thus, by mounting the two camerasrigidly with respect to each other and them performing an RCPcalibration, the system can be used to locate the wheels on one side ofthe vehicle with respect to the other side from that point on. This isto say that the RCP transfer function is used to convert one camera'scoordinate system into the other camera's coordinate system so that atarget viewed by one camera can be directly related to a target viewedby the other camera.

The illustrated inspection process is monocular, meaning that by usingone camera in one position, the position and orientation of a targetwith respect to the camera can be determined. This, of course, requiresthat the target be in view of the camera to accomplish the measurement.But since one camera can only conveniently view one side of the vehicleat a time without using reflectors as earlier described above, twospatially related cameras must be used to view both sides. The RCPtransfer function then allows the information obtained by the twocameras to be coordinated and have the same effect as if all of theinformation had been obtained by a single camera. An advantage of theuse of such a system is that, since each wheel is independentlyinspected and merely related back to the others, the system isindependent of level and does not require leveling of the vehiclesupport rack or floor. Moreover, it is not necessary that the axles ofall wheels be at the same height, i.e., differences in tire sizes orinflation will not adversely affect measurement.

In operation, once the system has been calibrated using the calibrationtarget 150 as illustrated in FIG. 9, a vehicle may be driven onto therack 133, and, if desired, the vehicle lifted to an appropriate repairelevation. The target assemblies 126 are then attached to the wheel rimsand manually oriented so that the target surfaces face the respectivecamera/light subsystems. The vehicle and model year are then enteredinto the keyboard 120 along with other relevant information which mayinclude the vehicle VIN number, license number, owner name, etc. Thesystem database includes specifications for each model that might beinspected, and upon identification of the particular vehicle underinspection extracts such information to assist in quickly locating thetarget images. Alternatively, previous inspection history can be used toindicate likely target location.

The targets are highly accurate and their position and orientationrelative to the rim of the wheel to which they are attached is known toan accuracy of 0.01" and 0.001°. If each wheel was perfect and the clampwas perfectly mounted one could argue that the wheel axle would benormal (90° in all directions) to the wheel plane determined by the rimedge. However, since wheels are normally not perfect and targets are notalways perfectly mounted, such information would only indicateorientation and position of the wheel plane and not necessarily provideaccurate information as to the orientation of the wheel axis. Suchassumption is thus not made. However, by rolling the wheel from oneposition to another a new image can be taken, and from the position andorientation of the target in the two positions, the actual position andorientation of the wheel axis can be calculated.

Similarly, to calculate the steering axle (about which the wheels turnwhen the steering wheel is turned) two target positions are againcompared, one with the wheels turned to one side and one with the wheelsturned to the other side. Calculation of the axis about which thetargets must have been moved thus identifies the position andorientation of the steering axis.

Now knowing where each wheel axle is located and how it is oriented,where the steering axles are located and how they are oriented, thevehicle can be mathematically modeled in three dimensions, and thealignment values in toe, camber, caster, thrust angle, etc. can bedisplayed with respect to the vehicle itself.

Once the targets are installed on each wheel and the system isenergized, enough information is available to generate an image such asthat depicted in FIG. 2a. However, as pointed out above, because therotational axis of the wheels may not be exactly normal to the wheelplane as defined by the outside perimeter of the rim to which the targetassembly is attached, the system operator will be instructed to move thevehicle forward or aft 6 or 8 inches so as to rotate the wheels throughabout 30° of rotation. With measurements taken of at least two differentwheel positions, the system can optically obtain enough information toaccurately determine true axle position and orientation for each wheel.Highly accurate computations can then be made and displayed on anupdated screen, as depicted in FIG. 2a.

At this point, the actual operator alignment procedure can proceed, andsince the inspection is continuous, the results of each adjustment willbe reflected on the system video screen. In the preferred embodiment ofthe present invention, the operator can select various levels ofassistance, including actual depictions of the location and parts to beadjusted to provide corrective action. Such information can even includethe appropriate choice of tool to be used.

As pointed out above, since each camera is referenced to the other, itis not necessary that the supporting rack be level or even that allwheels lie within the same plane. However, although each wheelinspection is independent of the others, a reference plane must beidentified. This can be accomplished by defining a reference plane thatpasses through the axles. But since one of the axles may not lie in theplane defined by the other three, some liberties must be taken. Forexample, for the purpose of aligning the front wheels, one might use theplane defined by the front axles and the average of the rear axles. Asimilar procedure might be used with respect to the rear wheels, etc.Wheel alignment would then be referenced to such a plane or planes. Inaddition, wheel position and thrust line measurements would also bereferenced to such a plane or planes. Moreover, because of theindependence of measurement, once a reference plane is defined, shouldone of the targets be blocked from view or become loose or evendislodged from a wheel, it will not necessarily affect measurementsassociated with other wheels.

Use of a single calibration target such as that illustrated at 150 inFIG. 9 may, however, in some respects be problematic. For example, theorientation of the two cameras 122, 124 must be set up so that bothcameras can see the same target; that is, the fields of view of thecameras must overlap at the position of the target 150. Since it isdesirable that the cameras have as narrow a field of view as possible,e.g., to optimize sensitivity and reduce optical distortion, the singletarget may have to be positioned a fairly substantial distance from thecameras so as to lie within the fields of view of both cameras.Therefore, it may be necessary to make the target large, unwieldy andexpensive.

However, according to the present invention, the above-described singletarget calibration disadvantages can be overcome through use of acalibration target assembly such as that depicted at 160 in FIGS.12a-12d. The illustrated assembly is comprised of a front target 162 anda rear target 164, respectively mounted in cantilevered fashion toopposite ends of a suitably configured rigid support structure formed bya two-piece telescoping beam 166, 167 mounted to upstanding support legs168 and 169 rigidly affixed thereto. The beam parts 166 and 167 may berigidly locked together at a desired extension by a suitable screw-clamp165 or the like. The lower end of leg 168 is provided with a base 170extending laterally to one side of the leg. Base 170 is provided with asupporting pad 171 disposed beneath its distal end. The lower end of leg169 is provided with a base 172 that extends to both sides of leg 169and has a support pad 173 disposed beneath one end thereof and a twoposition tilt mechanism 174 affixed to the other end. As will bediscussed below, the tilt mechanism 174 is a simple repositionablesupport that in one position causes the base 172 to lie horizontal, andin a second position causes the entire assembly to be tilted slightlyinto a new orientation to allow the checking of a previously calculatedcalibration.

Note that since targets 162 and 164 both extend laterally to the sameside of beam 166, 167, the center of gravity of the structure is to oneside of the beam 166, 167. Accordingly, the three supports 171, 173 and174 are positioned in unbalanced relationship to the beam centerline soas to counter the unbalance caused by the side-mounted targets.

Although the targets 162 and 164 are for simplicity shown with plainfaces, it will be understood that they are actually provided withoptically detectable geometric designs of the type described above. Thetarget faces are angularly disposed at typically 45° relative to thelongitudinal axis of beam 166, 167 for reasons which will be discussedbelow. Note that rear target 164 is somewhat larger than front target162 to compensate for the difference in distance to the camera.

Also affixed to beam part 167 are simple C-shaped handles 176 and 177that may be used to lift the assembly to carry or move it from oneposition to another.

In use, the assembly is normally extended as illustrated in FIG. 12a bythe solid lines; however, as suggested by the dashed lines, by releasingthe clamp 165, beam part 166 may be telescoped into beam part 167 topresent a more compact assembly for transport or storage.

Turning now to FIGS. 13 and 14, the improved calibration method of thepresent invention will be described using the assembly 160. With theauto lift rack 180 raised to a typical inspection level, and with theassembly 160 positioned as indicated on the left rack track or rail 181,both the front target 162 and rear target 164 will be within the fieldof view of camera 122. Neither target will necessarily lie within thefield of view of the other camera 124. Since the targets 162 and 164 arenow in positions similar to those of the left side targets previouslydisclosed with reference to FIG. 9, it will be appreciated that camera122 can optically illuminate and detect the positions and orientationsof the targets relative to its own position and optical axis, as well asto each other, by using the techniques described above. But at thispoint, the system is not yet "calibrated" because the precise positionand orientation of camera 124 relative to camera 122 is not yet known.However, now that the separation between targets 162 and 164 is nowknown and their angular relationship to each other is known, by simplylifting the assembly 160 and placing it in bridging relationship acrossthe rack tracks 181 and 182 as illustrated in FIG. 14, so that target164 now lies within the field of view of camera 122, and target 162 lieswithin the field of view of camera 124, the two cameras can respectivelydetermine their position and orientation relative to the targets 164 and162. And since, as indicated above, the separation and relativeorientation of the targets is both known and fixed, the system canaccurately compute the orientation and position of camera 124 relativeto camera 122.

The system is thus now calibrated. However, as a check the techniciancan simply reposition or reorient the assembly 160 on rack 180 and causethe system to recompute the relative camera positions. One way toaccomplish such reorientation is to simply extend or retract the tiltmechanism 174 into its other position; another is to move the assemblyback a foot or so on the rack. If there is no change in camera positioncomputation, the calibration may be considered accurately accomplished.This also insures that the assembly has remained rigid and dimensionallyunchanged during its repositioning.

Among the advantages of using this calibration method and apparatus areas follows:

(1) Smaller targets can be used (as compared to the previously describedsingle target procedure).

(2) The calibration targets can be located closer to the cameras,thereby improving the accuracy of the process.

(3) The camera fields of view do not have to overlap.

(4) The assembly need only be held rigid during the calibrationoperation; thereafter it can be collapsed into a smaller configuration.

(5) The assembly need not be built and maintained to a high tolerance onrelative target orientation and position; the structure's relativetarget position is field calibrated.

Although the targets illustrated in FIGS. 12-14 are depicted as planarin configuration, it is to be understood that any other suitable targetdesign could likewise be used so long as it includes features whichenable the position and orientation of the target to be opticallydetermined by the cameras 122, 124 or other optical inspection means.Moreover, the targets need not necessarily be mounted to one side of thebeam 160. For example, they could be mounted above or below the beam inany suitable manner that allows unobstructed viewing by the cameras whenthe assembly is disposed in the calibrating positions described above.However, one benefit of the disclosed structure is that the sametargets, or type of targets, used in the subsequent wheel alignmentoperations can be affixed to the beam 160 in the same manner and usingthe same fastening techniques used to mount the targets to the autowheels.

Having now described several embodiments of opto-electronic apparatussuitable for use in aligning the wheels of a vehicle, and having pointedout that the position and orientation of each target and associatedwheel may be determined independently of the other targets (and wheels),it will be appreciated by those skilled in the art that by modifying thetarget attachment structure to enable the targets to be affixed to otherparticular points on the vehicle, or to another type of structure, suchas, for example, a building structure, an article of manufacture, arobot arm, or even territorial space, the same system can be used tomeasure relative spatial location or alignment of the several points towhich the targets are affixed. For example, in the case of an automotivevehicle or the like, one might use the described system to measurevehicle chassis or body alignment, or perhaps ride height. And becausethe data is updated at a high rate, "jounce" measurements (i.e., ameasurement of suspension dynamics) can be made. In the case of articlesof manufacture, one might wish to embody a target in the form of a labeland affix the label to parts on an assembly line, and then use theapparatus to track the position and/or orientation of the article as itmoves down the line. In the case of a robot arm, one or more targetsaffixed to various moving parts could be used to accurately follow themotion of the arm as objects are carried thereby. In the case ofbuilding structures, one might use a system in accordance with thepresent invention to determine or maintain alignment of various pointson the structure relative to other points. In the case of territorialspace, one might use the system to develop topological surveys of groundsurface contours.

It will also be apparent that more than two cameras could be used toinspect objects or fields of view not readily inspectable with one ortwo cameras. In such case an RCP transfer function calibration proceduresimilar to that described above would be followed.

Additional Features

As indicated above, this apparatus can also be used to determine thecondition of the shock absorbers of the vehicle. This is done by firstly"jouncing" the vehicle. Jouncing a vehicle is a normal step in alignmentprocedures, or, for that matter, checking the shock absorbers, andentails applying a single vertical force to the vehicle by, eg. pushingdown onto the hood of the vehicle and releasing the vehicle, to cause itto oscillate up and down. Secondly, as the vehicle oscillates up anddown, the apparatus of the invention takes readings of the targets oneach of the wheels. In so doing, the movement of the targets, which willdefine a dampened waveform, can be monitored to determine the extent ofthe dampening. If the dampening is not sufficient (i.e. the up and downmovement or rocking of the vehicle does not stop soon enough) thisindicates that the shock absorbers are faulty.

This method is particularly advantageous in that a determination can bemade as to the soundness of a specific shock absorber; a result whichcan be indicated to the operator of the alignment apparatus by means ofthe computer 32.

It will be evident that in the determination of the condition of theshock absorbers of the vehicle, any suitable portion of the body of themotor vehicle can be selected to monitor the oscillation of the vehicle.So, for example, the apparatus can focus on the edge of the wheelhousing or, alternatively, a small target placed on a convenientposition on the body work of the motor vehicle.

In addition, this apparatus can be used to calculate the ride height ofthe motor vehicle. This parameter is particularly important in thedetermination of the alignment of the wheels of vehicles such aspick-ups which, in operation, may carry a load. This load would have theeffect of lowering the vehicle and it is, therefore, preferable to makeallowance for this during alignment procedures. Traditionally, the rideheight, or height of the chassis of the vehicle from the floor, isdetermined by physically measuring it with an instrument such as a tapemeasure. This measurement is then compared to standard tables whichyield a compensation factor for the vehicle concerned.

The described method and apparatus can, however, make this measurementdirectly by viewing an appropriate portion of the body and determiningits height from the test bed on which the vehicle rests. Once thisheight has been determined it can be compared to standard look-up tablesstored within the computer which can, in turn, produce the compensationfactor.

Advantages

A general advantage of the above-described apparatus is that it isrelatively simple to use as no delicate mechanical or electronicequipment need be attached to the wheels of the motor vehicle concerned.As the sensitive and delicate equipment is mounted within a housingwhich stands independent and distant from the motor vehicle beingtested, no damage can be caused to it if the motor vehicle were, forexample, to be driven off the wheel guides. Whereas prior art heads canbe knocked out of calibration by simple jarring or dropping, it takesmajor damage to the wheel-mounted components to affect the calculatedresults.

Another advantage is that the equipment requires very few operatorcommands and could readily be made hands free with simple auditoryoutputs and equally simple voice recognition means to receive and/orrecord operator responses and/or commands.

The described system has the further advantage that alignmentdeterminations can be done relatively quickly. This allows a higher turnaround rate within the business conducting the alignment determinations.

A still further advantage of this apparatus is that it can be placed, asis illustrated in FIG. 4, above and out of the way of the motor vehiclebeing tested. This has the distinct advantage that the chances ofdamaging the sensitive alignment determining apparatus is substantiallyreduced as the apparatus is out of the way of the motor vehicle. Anotheradvantage of this configuration is that the measuring apparatus usesminimal floor space and has no equipment blocking access to the front ofthe motor vehicle.

Furthermore, as the vehicle can be backed up and driven forward, thisapparatus has the advantage that it is unnecessary to jack the vehicleup to make the required calculations for "run-out". In addition, thisapparatus can be used to determine information other than the relativealignment of the wheels. For example, the alignment apparatus, ifequipped with a suitable character recognition capability, could be usedto read the license plate of the motor vehicle which could, in turn,yield information such as the make and model of the vehicle and itsservice history (if available) and, therefore, the required alignmentparameters of such vehicle. This would save the operator from enteringthe motor vehicle's details into the apparatus. As more manufacturersare adding bar codes to the VIN number plates, similar information canalso be obtained by optically viewing and processing the bar-codedplate. In addition, it would also be possible to optically identify thevehicle type by comparing certain features of the body or trim thereofto database information.

Yet another advantage of the system is that no wires, cords or beams oflight pass in front of the vehicle being tested. As most alignmentcorrection is made by accessing the wheels of the car from the front,wires, cords or beams passing in front of the vehicle tend to get in theway of the technician. Often these wires, cords or beams are sensitiveto being interfered with and so their absence makes alignment correctionwork much easier.

Related to this advantage is the fact that there are no cords or wirespassing between the targets on the wheels, nor are there any wiressupplying power to the targets from a remote power source. This absenceof wires or cords once again makes work on the vehicle easier.

In addition, as the targets are not interlinked or interdependent, afterinitial capture of target images, it is possible to block off one of thetargets from the camera's view without interfering with the orientationcalculations for the other wheels. In the prior art devices describedearlier, all the test heads are interdependent and cannot function ifone of the heads is "blocked" out.

It will be evident to those skilled in the art that the concept of thissystem can be applied in many different ways to determine the alignmentof the wheels of a motor vehicle. So, for example, the apparatus coulddefine a reference point for each wheel with the referent point beinglocated at, say, the intersection of the axis of rotation of the wheel,with that wheel. These points can then be processed to define anapproximately horizontal reference plane, relative to which thealignment of the wheels can be calculated.

This method has the particular advantage that the rack on which thevehicle is being supported does not have to be levelled, a process whichrequires expensive apparatus and which is necessary to define ahorizontal reference plane and which is used in prior art alignmentdevices.

While the present invention has been particularly shown and describedabove with reference to a preferred embodiment, it will be understood bythose skilled in the art that various alterations and modifications inform and in detail may be made therein. Accordingly, it is intended thatthe following claims be interpreted as covering all such alterations andmodifications as may fall within the true spirit and scope of theinvention.

What is claimed is:
 1. Apparatus for use in calibrating anopto-electronic alignment system including at least first and secondfixed and spaced apart optical inspection devices and an associated dataprocessor adapted to optically inspect and determine the position andorientation relative to said inspection devices of targets affixed toobjects such as vehicle wheels to be aligned, and from suchdetermination to infer the position and orientation of the objects towhich the targets are affixed, comprising:means forming an elongatedbeam; support means for supporting said beam at a predeterminedelevation above a work surface; and first and second target meansrigidly affixed to said beam at spaced apart locations, each said targetmeans having predetermined geometric target features formed on a targetface thereof, said target faces being oriented at predetermined fixedangles relative to the length of said beam such that when the length ofsaid beam is disposed to extend generally normal to an imaginary lineconnecting said first and second inspection devices, said first andsecond target faces lie within the field of view of said firstinspection device, and when the length of said beam is disposed toextend generally parallel to said line, said first target face lieswithin the field of view of said first inspection device and said secondtarget face lies within the field of view of said second inspectiondevice.
 2. Apparatus as recited in claim 1 wherein said target faces aregenerally planar and are oriented at predetermined angles relative tothe length of said beam.
 3. Apparatus as recited in claim 2 wherein eachsaid target face is oriented at an angle of substantially 45° relativeto the length of said beam.
 4. Apparatus as recited in claim 1 whereinsaid support means includes a pair of vertically oriented support legshaving one end rigidly affixed to said beam and opposite ends affixed tobase means adapted to support said beam and the affixed target means. 5.Apparatus as recited in claim 1 wherein said first and second targetmeans are both disposed on one side of said beam.
 6. Apparatus asrecited in claim 1 wherein said beam is a two-part mechanism having onepart telescopically receivable within the other so as to betelescopically collapsible for storage and telescopically extendible forcalibration use.
 7. A method of calibrating an opto-electronic alignmentsystem including at least first and second spaced apart opticalinspection devices and an associated data processor adapted to inspectand determine the position and orientation relative to said inspectiondevices of targets affixed to objects such as vehicle wheels to bealigned, and from such determination to infer the position andorientation of the objects to which the targets are affixed, comprisingthe steps of:providing first and second target means; providing a meansfor supporting said first and second target means in rigid dispositionrelative to each other and at a predetermined fixed distance from eachother; disposing said supporting means relative to a work surface suchthat both said first and second target means lie within the field ofview of said first inspection device; optically inspecting said firstand second target means to obtain first optical information useful todetermine the position and orientation of each said target meansrelative to said first inspection device; reorienting said supportingmeans such that said first target means lies within the field of view ofsaid first inspection device and said second target means lies withinthe field of view of said second inspection device, and again opticallyinspecting said first and second target means to obtain second opticalinformation useful to determine the present position and orientation ofsaid first target means relative to said first inspection device and thepresent position and orientation of said second target means relative tosaid second inspection device; and using the first and second opticalinformation to compute the precise position and orientation of saidsecond inspection device relative to said first inspection means.
 8. Amethod as recited in claim 7 wherein said first and second target meanshave target faces that are generally planar and have predeterminedangular orientation relative to said supporting means.
 9. A method asrecited in claim 8 wherein each said target face is oriented atsubstantially 45° relative to an imaginary line connecting said firstand second target means.
 10. Apparatus for calibrating anopto-electronic alignment system including at least first and secondspaced apart optical inspection devices and an associated data processoradapted to inspect and determine the position and orientation relativeto said inspection devices of targets affixed to objects such as vehiclewheels to be aligned, and from such determination to infer the positionand orientation of the objects to which the targets are affixed,comprising:first and second targets each having predetermined geometricattributes that can be optically detracted; a support for supportingsaid first and second targets in rigid disposition relative to eachother and at a predetermined fixed distance from each other so that whensaid support is disposed relative to a work surface such that both saidfirst and second targets lie within the field of view of said firstinspection device, the targets can be optically inspected by thealignment system to obtain first optical information useful to determinethe position and orientation of each said target relative to said firstinspection device, and when said support is reoriented such that saidfirst target lies within the field of view of said first inspectiondevice and said second target lies within the field of view of saidsecond inspection device, the targets can be optically inspected by thealignment system to obtain second optical information useful todetermine the present position and orientation of said first targetrelative to said first inspection device and the present position andorientation of said second target relative to said second inspectiondevice, and the first and second optical information can be used tocompute the precise position and orientation of said second inspectiondevice relative to said first inspection device.
 11. Apparatus asrecited in claim 10 wherein said first and second targets have targetfaces that are generally planar and have predetermined angularorientation relative to said support.
 12. Apparatus as recited in claim11 wherein each said target face is oriented at substantially 45°relative to an imaginary line connecting said first and second targets.